The use of diagnostic ultrasound in medicine has expanded enormously over the past few years and has become a vital clinical tool in the diagnosis and assessment of a large variety of ailments and conditions. There has been a steady evolution in the complexity and sophistication of the imaging and Doppler modalities that provide the principal instrumental tools for clinicians. These devices have tended, in general, to evolve to higher working frequencies to improve resolution, and consequently, because of the higher attenuation, to larger acoustic intensities and pressure amplitudes. There are two principal mechanisms whereby diagnostic ultrasound can interact with tissue with the potential for a bioeffect: a) a thermal mechanism resulting from absorption of the ultrasonic pressure wave and subsequent temperature elevation of the tissue; b) the phenomenon of acoustic cavitation in which relatively small amounts of energy are concentrated to microscopic scales, thus producing intense localized pressure and temperature. Concern for these potentialities has prompted concerted action by the FDA, American Institute for Ultrasound in Medicine, and NEMA to draft Output Display Standards for Real-Time Display of Thermal and Mechanical Indices. The current mechanical index (MI) is based on in vitro tests of the acoustical conditions for bubble nucleation in liquids containing prescribed bubble nuclei (polystyrene spheres and albumin-coated bubbles), as well as a theoretical model of the interaction of ultrasound with materials. Yet this work, though involving careful experimentation and theoretical efforts by many independent researchers, still cannot answer the question: Does diagnostic ultrasound produce bubbles in human tissue? The question has not been answered definitively because acoustical detection of micron-sized bubbles that last a microsecond is an extremely difficult task, especially if this transient, weak event is buried in the backscattered noise from blood cells, tissue inhomogeneities, etc. The proposed effort will couple the most sensitive bubble detection system with advanced signal processing techniques to determine the minimum conditions for acoustic cavitation inception in vivo. Of equal importance, yet not fully addressed thus far, is the question of the role played by the pulse duration and duty cycle of the acoustical imaging or doppler signal in determining bubble nucleation and activity, the latter being related, in all likelihood, to the degree of bioeffect that is possible when the mechanical index is exceeded. Thus, a systematic study of these parameter regimes is envisioned, for liquids, blood, and tissue equivalent phantoms. The above work on in vivo detection and duty cycle will be complemented by a continuation of studies of bioeffects induced in epithelia of frogs by diagnostic and therapeutic ultrasound, in order to understand better the formation of free radicals and the changes in ionic conductance that can potentially influence intracellular components.